Requests already counted in this headless run, keyed by message ID.
Monotonic across HITL resume passes so a replayed message does not add its
request, tokens, or cost to stats again. Each pass closes its entries via
finalize_recorded_requests, which is what extends that guarantee to
replayed chunks -- an open chunked request accepts revisions, so without
the round boundary a replayed chunk would merge into it a second time.
